The present invention relates to a device for monitoring an area, which device comprises a transmitter unit for transmitting a signal and a receiver unit for receiving a reflected signal via an antenna.
The installation of a monitoring device in the form of an active sensor for monitoring an area in front of a vehicle is already known. The sensor has a poor range and the radiation from the sensor in the area that is to be monitored can be regarded as originating from a point source. When the sensor is located in the centre of the front of the vehicle, acceptable values can be obtained for the relative speed between the vehicle and a reflecting object that is centrally in front of the vehicle in the direction of movement of the vehicle. However, if the reflecting object is displaced sideways relative to the direction of movement of the vehicle, an unacceptable error in the speed can easily arise. There is also the risk that the object will leave the sensor's field of vision when there is a short distance between the reflecting object and the vehicle. In order to increase the accuracy when determining speed, it would be possible to install several individual sensors along the front of the vehicle. The use of several sensors with associated arrangement for signal processing and installation means that the monitoring device runs the risk of being both complex and expensive.
A monitoring device in the form of a security alarm is already known from U.S. Pat. No. 3,947,834. The alarm is provided with a combined transmitting and receiving antenna in the form of a cable with groups of slots. The antenna is supplied at one end with a radar frequency which, when it is reflected from moving objects, gives rise to a Doppler frequency signal which under certain conditions causes the alarm to be activated. The security alarm detects movement but carries out no direct determination of speed or distance. Due to the not insignificant suppression that is introduced along the length of the antenna, the antenna has different sensitivity along its length, with greater sensitivity close to the supply end. In addition, the length of the signal wave varies in the antenna. Reflections close to the supply end of the antenna have a shorter signal wave in the antenna than reflections close to the end of the antenna that is furthest away from the supply end. Among other things, the circumstances described above make it difficult, if not impossible, to determine the relative speed and distance of an object with any accuracy, if so required.
A device according to the first section of the description is previously known from U.S. Pat. No. 4,612,536. In this case a security alarm is again in question. The intrusion detector of the security alarm here comprises two parallel antenna cables preferably located in the ground in connection to an object to be protected. The antenna cables are in opposite ends connected to a transmitter unit and a receiver unit, respectively and the asymmetry occurring in the transmitter antenna lobe due to attenuation along the cable will be balanced out of a corresponding reversed asymmetry in the antenna lobe of the receiver antenna. The intention is to detect objects if any passing above the antenna cables and if desired to determine the location of the passage. However, there is no determining of the speed of an object approaching the object to be protected.